SAS 3 and Einstein observations of the 11 minute orbital period of the globular cluster X-ray source 4U 1820-30

Details SAS 3 and Einstein observations of the 11 minute orbital period of the globular cluster X-ray source 4U 1820-30 SAS 3 and Einstein observations of the 11 minute orbital period of the globular cluster X-ray source 4U 1820-30

Jan 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988apj...324..851m&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 324, Jan. 15, 1988, p. 851-858.

Other

27

Globular Clusters, Heao 2, Periodic Variations, Sas-3, X Ray Binaries, Accretion Disks, Dwarf Stars, Exosat Satellite, Light Curve, Neutron Stars, Stellar Mass Accretion

Scientific paper

We confirm the 685 s modulation of the X-ray intensity of 4U 1820-30, a bright X-ray source in the globular cluster NGC 6624. A unique period is determined from 15 observations made with SAS 3 during 1976 and 1977. The amplitude of the modulation (peak-to-peak) is seen to vary between 1% and 5% in the energy range of 1-6 keV, and the average modulation amplitude is 2.6%. In each case, the X-ray source was in a high-luminosity, non-bursting state, and we find no correlation between the modulation amplitude and the source luminosity. The period determined by combining the SAS 3 epochs with those from EXOSAT and the Einstein Observatory is 685.011836(32) s. The upper limit (3 σ) on the period derivative is -0.09

1.2x10-7 yr-1. The cause of the X-ray modulation is not uniquely determined, but comparisons with other X-ray light curves favors the partial obscuration of an extended X-ray source by a bulge in the accretion disk.

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